“Direct” Gas-phase Metallicity in Local Analogs of High-redshift Galaxies: Empirical Metallicity Calibrations for High-redshift Star-forming Galaxies

Abstract

We study the direct gas-phase oxygen abundance using the well-detected auroral line [O III]λ4363 in the stacked spectra of a sample of local analogs of high-redshift galaxies. These local analogs share the same location as z ∼ 2 star-forming galaxies on the [O III]λ5007/Hβ versus [N II]λ6584/Hα Baldwin-Phillips-Terlevich diagram. This type of analog has the same ionized interstellar medium (ISM) properties as high-redshift galaxies. We establish empirical metallicity calibrations between the direct gas-phase oxygen abundances (7.8 < 12 + log(O/H) < 8.4) and the N2 (log([N II]λ6584/Hα))/O3N2 (log(([O III]λ5007/Hβ)/([N II]λ6584/Hα))) indices in our local analogs. We find significant systematic offsets between the metallicity calibrations for our local analogs of high-redshift galaxies and those derived from the local H II regions and a sample of local reference galaxies selected from the Sloan Digital Sky Survey (SDSS). The N2 and O3N2 metallicities will be underestimated by 0.05-0.1 dex relative to our calibration, if one simply applies the local metallicity calibration in previous studies to highredshift galaxies. Local metallicity calibrations also cause discrepancies of metallicity measurements in high-redshift galaxies using the N2 and O3N2 indicators. In contrast, our new calibrations produce consistent metallicities between these two indicators. We also derive metallicity calibrations for R23 (log (([O III]λλ4959,5007+[O II]λλ3726,3729)/Hβ)), O32(log([O III]λλ4959,5007/[O II]λλ3726,3729)), log([O III]λ5007/Hβ), and log([Ne III]λ3869/[O II]λ3727) indices in our local analogs, which show significant offset compared to those in the SDSS reference galaxies. By comparing with MAPPINGS photoionization models, the different empirical metallicity calibration relations in the local analogs and the SDSS reference galaxies can be shown to be primarily due to the change of ionized ISM conditions. Assuming that temperature structure variations are minimal and ISM conditions do not change dramatically from z ∼ 2 to z ∼ 5, these empirical calibrations can be used to measure relative metallicities in galaxies with the redshifts up to z ∼ 5.0 in ground-based observations.

VLT/X-Shooter Spectroscopy of Lyman Break Analogs: Direct-method O/H Abundances and Nitrogen Enhancements

“…The relatively high ionization parameters, relatively low oxygen abundances, excess N/O and BPT offsets are of a similar order of magnitude as those observed by other authors for star-forming galaxies at z ∼ 2 − 3 as well as previously studied local analogs (e.g. Amorín et al 2010; 2014; Masters et al 2016;Sanders et al 2016a;Steidel et al 2014;Steidel et al 2016;Strom et al 2017;Kojima et al 2017;Bian et al 2018;Strom et al 2018).…”

“…3.2 versus the O3N2 oxygen abundance from Pettini & Pagel (2004). The relations found by Bian et al (2018) for typical SDSS star-forming galaxies (red dashed line) and their sample of BPT offset analogs (blue solid line) are indicated. Right: Direct method oxygen abundance determined in Sect.…”

“…. The relations found byBian et al (2018) for typical SDSS star-forming galaxies (red dashed line) and their sample of BPT offset analogs (blue solid line) are indicated. In both figures, the direct-method abundances derived using the Te(O II) derived from RO2 are shown as open circles, while those derived using Te(O II) as inferred from Te(O III) are shown as filled circles.…”

What drives the redshift evolution of strong emission line ratios?

“…The analogues of high-redshift galaxies and the SDSS reference galaxies have been divided into seven bins of 0.25 dex in the range −2.25 to −0.25 in their [N ii]/Hα ratios. We refer the readers to Table 3 and Table 4 in Bian et al (2018) for detailed properties of galaxies in each of the [N ii]/Hα bins. We generate the stacked spectra in each [N ii]/Hα bin as follows.…”

“…The He iiλ4686 lines are all well detected (S/N > 10) in all the stacking spectra. We refer readers to Section 3 in Bian et al (2018) for more detail for the procedure on stacked spectra, subtracting the stellar continuum, and emission-line fitting.…”